Mold clamping apparatus in an injection molding machine

ABSTRACT

A mold clamping apparatus having a plurality of servomotors for drivingly rotating a ball screw as a spindle. A driven timing gear pulley is mounted on a ball nut threadedly engaged with the ball screw whereby the pulley is rotatable integrally with the nut, and driving timing gear pulleys are fixed to output shafts of the respective motors. A timing belt is wound between the driven and driving pulleys. The servomotors are driven so as to produce the same torque by means of driving circuits which respond to the same torque command generated in accordance with a signal representative of a rotational position of one of the servomotors.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a mold clamping apparatus for use in aninjection molding machine, and more particularly to a spindle drivingdevice in a mold clamping apparatus, which is low in cost and has animproved responsiveness.

2. Description of the Related Art

In injection molding machines, mold clamping apparatus have been knownof a type comprising a drive source consisting of an electric motor(hereinafter referred to as a motor), and a power transmission systemfor converting a turning force generated by the drive source into adriving force for axially driving a spindle, and wherein a movableplaten arranged to be movable in unison with the spindle is axiallydriven by the drive source through the power transmission system so asto perform mold-opening, mold-closing and mold-clamping. In a moldclamping apparatus of this kind, the drive source may consist of asingle general-purpose motor capable of generating a required moldclamping force of from 5 to 10 tons in case of a small or mediumapparatus. In a large mold clamping apparatus requiring a larger moldclamping force, however, the drive source must be of a specially orderedhigh-output type. For this reason, a considerable cost increase occursand further operational responsiveness of the motor deteriorates sincethe inertia of the rotor becomes large.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a mold clampingapparatus for use in an injection molding machine, which is low in costand has an improved responsiveness.

In order to achieve the above-mentioned object, the mold clampingapparatus of the present invention comprises a plurlity ofgeneral-purpose motors, each coupled through a coupling means to aspindle of the mold clamping apparatus, so as to drive the spindle bymeans of the plurality of motors.

Since the spindle of the mold clamping apparatus is adapted to be drivenby means of a plurality of the motors, the spindle driving device of themold clamping apparatus is low in cost. Moreover since the rotor of eachmotor is relatively small in diameter the sum of the rotor inertia ofthe plural motors is smaller than that of a single special motor havinga large rotor. Hence the operational responsiveness of the motor, inparticular, the acceleration/deceleration characteristics thereof can beimproved.

These, together with other objections and advantages, which will besubsequently apparent, reside in the details of construction andoperation as more fully hereinafter described and claimed, referencebeing made to the accompanying drawings forming a part thereof, whereinlike numerals refer to like parts throughout.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspectie view showing a spindle driving device of a moldclamping apparatus, according to an embodiment of the present invention;

FIG. 2 is a perspective view showing, by way of example, a conventionalspindle driving device; and

FIG. 3 is a circuit diagram showing a control unit for driving the motorof FIG. 1.

DESCRIPTION OF THE REFERRED EMBODIMENT

Referring to FIG. 2, a conventional spindle driving device for use in amold clamping apparatus of an injection molding machine will beexplained. In FIG. 2, a ball screw 2 which constitutes a spindle isrotatably supported by a rear platen 1, and a ball nut 3 is threadedlyengaged with an outer portion of the ball screw 2 projecting rearwardlyof the rear platen 1. A timing pulley (hereinafter referred to as apulley) 4 is coupled to the ball nut 3 in a manner so as to be rotatableintegrally with the ball nut. Further, the pulley 4 is operativelycoupled to a servomotor 5 through a timing belt 7 which is wound aroundthe pulley 4 and a second pulley 6 fixed to the output shaft of themotor 5. The ball screw 2 has its inner end coupled to a movable platen(not shown) through a crosshead (not shown) of the mold clampingapparatus (in the case where the apparatus has a toggle mechanism) orcoupled directly thereto (in the case of a direct-acting type clampingapparatus).

Where a single motor is employed as the drive source for mold clampingin the aforementioned type of mold clamping apparatus of an injectionmolding machine, there occur the above-mentioned drawbacks such asincreased cost of the spindle driving device, irrespective of whetherthe single motor consists of a servomotor or not.

In the following, a spindle driving device of a mold clamping apparatusfor use in an injection molding machine, according to an embodiment ofthe present invention, will be explained with reference to FIG. 1.Basically, this spindle driving device is constructed in a mannersimilar to that shown in FIG. 2, parts common in FIGS. 1 and 2 beingshown by likee reference numerals or symbols.

Symbols M1 and M2 designate spindle driving motors, each of whichcomprises an AC servomotor consisting of a general-purposepermanent-magnet synchronous motor. These motors are fixed integrally tothe rear platen 1 by an appropriate means, not shown, and each have anoutput shaft to which drive pulleys 8 and 9 are fixed, respectively.Further, a driven pulley 4 is fixed to a ball nut 3 which is threadedlyengaged with a ball screw 2, which constitutes the single spindle of themold clamping apparatus. The motors, M1, M2 are operatively coupled tothe ball nut 3 through the pulleys 4, 8 and 9 and a timing belt 7 iswound around these pulleys.

Furthermore, in order to produce, from the two general-purpose motors M1and M2, a combined output equivalent to that of a single high-outputtype motor, the mold clamping device according to this embodiment isarranged to drive these motors so that the motors M1, M2 are driven insynchronism with each other to produce their output torque acting in thesame rotational direction. More specifically, a control unit isprovided, which has a control circuit responsive to a position/speedsignal associated with one of the motors, the motor M1, for instance,and two motor driving circuits for the motors M1 and M2, the controlunit being arranged to deliver the same drive current command from itscontrol circuit to both of the motor drive circuits.

Next, with reference to FIG. 3, a further explanation as to the controlunit will be given. Basically, this control unit is constructed in thesame manner as a conventional PWM (pulse width modulation) control unitknown as a servomotor driving control unit, but differs from aconventional one in that two transistor inverters as power amplifiersare provided, these inverters being connected in parallel with eachother. In FIG. 3, reference symbol E designates a three-phase powersource, numeral 10 denotes a rectifier, and 11 a transistor inverter, asa power amplifier, which generates an output for driving the servomotorM1. An ordinary PWM control unit only has a transistor inverter 11 fordriving a single servomotor. Contrary to this, in the presentembodiment, a further transistor inverter 11' is connected in parallelwith the transistor inerter 11 and is arranged to drive a correspondingservomotor M2. Reference numeral 12 denotes a transistor PWM controlcircuit, and symbol P designates the position detector, such as a pulseencoder, for detecting the position and speed of the rotor of theservomotor M1.

The transistor PWM control circuit 12 is arranged to compare a presentspeed S, detected by the position detector P, with a speed command Vofrom a host control unit such as an NC (numericaly controlled machine),to turn on and off transistors TA to TF and TA' to TF' of the transistorinverters 11 and 11', so as to control currents flowing through the U-,V-, and W-phase windings of the servomotores M1 and M2, therebycontrolling the rotating speed of the servomotors M1 and M2. Namely, inresponse to PWM signals PA and PB delivered from the transistor PWMcontrol circuit 12, the transistors TA and TA' and the transistors TBand TB' of the transistor inverters 11 and 11' are simultaneously turnedon or off, whereby the currents flowing through the respective U-phasewindings of the servomotors M1 and M2 are controlled. Likewise, inresponse to PWM signals PC and PD, the transistors TC, TC' and thetransistors TD, TD' are simultaneously turned on or off, whereby thecurrents flowing through the respective V-phase windings of theservomotors M1, M2 are controlled. In response to the PWM signals PE andPF, moreover, the transistors TE, TE' and the transistors TF, TF' aresimultaneously turned on or off, whereby the currents flowing throughthe respective W-phase windings of the servomotors M1, M2 arecontrolled. As a result, the servomotors M1 and M2 are driven insynchronism with each other so as to produce the same torque. A detaileddescription of the transistor PWM control circuit 12, which is aconventional one, is omitted herein.

In operation, when the speed command value Vo from the host control unitsuch as an NC, is applied to the transistor PWM control circuit 12, thecircuit 12 compares the speed command value Vo with the present speed Sdetected by the position detector P. In accordance with the differencebetween the value Vo and the speed S, the control circuit 12 deliversthe PWM signals PA to PF, and causes the transistor inverters 11 and 11'to drive the servomotors M1 and M2, so as to control the motor speeds toa command speed. At this time, the transistors associated with thecorresponding phases of the transistor inverters 11, 11', serving tocontrol the currents flowing through the windings of the individualphases of the servomotors M1 and M2, are turned on or off in accordancewith the same PWM signal, so that one and the same current flowssynchronously through the windings of each phase of the servomotors M1and M2. In consequence, the servomotors M1, M2 rotate in synchronismwith each other, with the same output torque, to drive the timing belt7. In the meantime, since the two timing gear pulleys 8 and 9 are inengagement with the timing belt 7, respectively, no phase dislocationoccurs between these timing gear pulleys.

In connecting the motors M1, M2 with the ball screw or spindle 2 a geartrain may be employed in place of the timing belt. Further, three ormore motors may be employed as distinct from the above-mentionedembodiment having two motors. In this case, it is only required toprovide additional transistor inverters which correspond in number tothe additional servomotors, and which are connected in parallel with thetransistor inverters 11, 11'. Hence neither an additional positiondetector P such as a pulse encoder nor an additional transistor PWMcontrol unit is required. Although an AC servomotor consisting of apermanent-magnetic synchronous motor is employed in the abovementionedembodiment, a DC servomotor may be used. In this case, one or moreadditional power amplifiers alone should be provided in the servomotorcontrol unit, while other control circuits and position detectors may becommonly employed.

Numerous alterations and modifications of the structure herein disclosedwill suggest themselves to those skilled in the art. It is to beunderstood, however, that the present disclosure relates to thepreferred embodiments of the invention which is for purposes ofillustration only and is not to be construed as a limitation of theinvention. All such modifications which do not depart from the spirit ofthe invention are intended to be included within the scope of theappended claims.

We claim:
 1. A mold clamping apparatus having a fixed platen, a movableplaten and a spindle operatively connected to the moving platen, for usein an injection molding machine which is controlled by a hostcontroller, comprising:a plurality of servomotors for driving thespindle; coupling means for coupling each of said servomotors to thespindle; detecting means for detecting at least one of a rotationalposition and a rotational speed of one of said servomotors and producingan output signal indicative of said detected position or speed; acontrol circuit for generating a torque command signal in accordancewith a difference between said output signal from said detecting meansindicative of the detected position or speed and a command signal fromthe host controller; and driving circuits for driving correspondingservomotors, in accordance with said torque command signal supplied fromsaid control cicuit, said driving circuits corresponding in number tothat of said plurality of servomotors.
 2. A mold clamping apparatusaccording to claim 1, wherein the spindle comprises a ball screw and anoutput shaft extends from each motor, and wherein said coupling meansincludes a ball nut threadedly engaged with said ball screw, a driventiming gear pulley coupled to said ball nut to be rotatable integrallywith said ball nut, a driving timing gear pulley fixed to each outputshaft, and a timing belt wound between said driven timing gear pulleysand said driving timing gear pulleys.
 3. A mold clamping apparatusaccording to claim 1, wherein each of said servomotors has windings ofrespective phases and said driving circuits have switching elementswhich respectively control electric currents flowing through thewindings of the respective phases of said servomotors, and wherein saidcontrol circuit performs on and off control of said switching elementsin accordance with the difference between the output signal and thecommand signal, so that the same electric current flows through therespective windings which are associated with the same phase.